So I wonder... when it's night I obviosly don't see very well (especially with Nevada Skies darker nights). But it doesn't limit my character's perceptions. How about the NPCs? Can I sneak better at night time and at dark places? Probobly not?

I think darkness affects how well NPCs can see you while you're sneaking. Meaning if its dark they're going to have a bad time

If it's dark the NPCs won't be able to see you properly, as long as you hide between the shadows.